WO1989011175A1 - Protective power variator - Google Patents

Abstract

A variator circuit comprises a triac (4) mounted in series with a load (1) to be controlled, and a CTR variable series phase-shifter circuit (8, 9, 10). The point (11) midway between the resistive element (8, 9) and the capacitive element (10) of said circuit (8, 9, 10) is connected to the gate (6) of the triac. Connected in series in the branch of the circuit comprising the resistive element and the connection of said midway point to the triac is a non-linear component (18) the resistance of which can change from a minimum value to a maximum value when the current flowing through it increases by a minimum value corresponding to the normal operating conditions of the phase-shifter circuit to a maximum value corresponding to a short-circuit condition of the triac between the gate and the power electrode, said minimum value of the resistance being a value which has no appreciable effect on the functioning of the phase-shifter circuit and said maximum value of the resistance being a value which limits the current in the resistive element to a value which prevents any destruction or deterioration of the latter. Said non-linear component may be a CTR resistor, a fast safety fuse or, very advantageously, the filament of an incandescent lamp, the characteristics of said filament, in particular its equivalent resistance under nominal operating conditions, being chosen so that it volatilizes in the event of a short-circuit condition of the triac between the gate and the power electrode.

Description

 Safety power variator

The present invention relates to power dimming circuits, in particular light dimming circuits used for controlling the light intensity of incandescent lamps such as halogen lamps.

Conventionally, these dimmer circuits include a triac mounted in series with a charge to be controlled (the incandescent lamp, in the case of a light dimmer), and a variable series RC phase-shifter circuit whose midpoint between the element resistive and the capacitive element is connected to the trigger of the triac.

More specifically, the resistive element generally consists of a variable resistance (used to adjust the light intensity by the user) connected in series with a fixed resistance of lower value, so that the total resistance of the en¬ never seems to be zero, even for the minimum setting of the variable resistance, thus avoiding excessive trigger current.

The connection between the midpoint of the series RC circuit and the trigger of the triac is usually carried out via a diac forming a component with a triggering threshold, possibly in series with a Zener diode to allow the complete extinction of the circuit. power below a certain variable resistance setting threshold.

Both for domestic and professional uses, these drives must meet a certain number of safety standards, in particular the CEI 65 standard (French standard NF C92-130) which impose a certain number of severe safety conditions both in which concerns mechanical robustness (tensile strength on the wires, solidity of the housing, etc.) than electrical.

In particular, among the electrical constraints, the various short-circuits which can occur between the electrodes of the triac must not lead to damaging consequences following the production of sparks, excessive heating, etc. However, these short-circuits, which result from manufacturing defects tion of components, are not uncommon and, statistically, may not appear before several tens of hours of operation¬ ment only.

Among the three possibilities of short circuits between electrodes of the triac (which includes three electrodes, namely a trigger and two power electrodes), two of the short circuits will not have harmful consequences: this is the short -circuit between the two power electrodes (the lamp will then be supplied at full power, continuously, the dimmer having no effect) and short circuit between the trigger and those of the power electrodes which is connected to the sector on the same side as the resistive element of the phase shifting circuit (the loop of the trigger control circuit will then short-circuit on itself, the triac will no longer be triggered, and the variator will also have no effect). On the other hand, the third short-circuit condition can, in certain cases, have serious damaging consequences.

Indeed, in the event of a short circuit between the trigger and those of the power electrodes which is connected to the sector on the same side as the capacitive element of the phase-shifting circuit, the midpoint of the phase-shifting circuit between the resistive element and the element capacitive will be brought to the potential of the mains phase at the location of the short circuit, so that the resistive element of the phase shifting circuit will be subjected between its terminals to a voltage equal to the mains voltage. This mains voltage can induce an intense current in the variable resistance and in the fixed resistance, a current which will be all the more intense as the variable resistance will be positioned on a low value. The resulting current can cause destruction of the fixed resistance or of the variable resistance, or of these two elements, with a heating such that this destruction can be accompanied by an emission of smoke and flames by combustion of the printed circuit board supporting these circuit elements.

The object of the present invention is to provide a power variator circuit of the aforementioned type which provides complete security against such an operating incident, thus ensuring total compliance with the aforementioned security standards. matter.

To this end, according to the invention, the power variator circuit comprises, in series in the circuit branch comprising the resistive element and the connection of said midpoint to the triac, a non-linear component whose resistance is susceptible to go from a minimum value to a maximum value when the current flowing through it increases respectively from a minimum value corresponding to the normal operating conditions of the phase shifter circuit to a maximum value corresponding to a triac short-circuit condition between trigger and power electrode, said minimum resistance value being a value without appreciable effect on the operation of the phase shifting circuit and said maximum resistance value being a value limiting the current in the resistive element to a value preventing any destruction or deterioration of that -this.

In a first embodiment, said non-linear component is a PTC resistor; in another embodiment, said non-linear component is a fusible element.

In a first variant of implementation of this second embodiment, the fuse element is a fast safety fuse; in a second variant of implementation - which is the preferred variant of implementation of the invention - the fusible element consists of the filament of an incandescent bulb, a filament whose characteristics, in particular its equivalent resistance under nominal operating conditions, are chosen so that it volatilizes in the event of a triac short-circuit condition between trigger and power electrode.

Other characteristics and advantages of the present invention will appear better on reading the detailed description of an exemplary embodiment, made with reference to the single appended figure, which represents the diagram of a power variator circuit implementing the teachings of the present invention. In this figure, the reference 1 designates the load to be controlled (typically, an incandescent lamp such as a halogen lamp), which is supplied by the two phases 2 and 2 ′, phase 2 being connected directly to the load, while that phase 2 'is connected with the interposition of a general fuse 3, a triac 4 and a switch 5, for example a foot-operated switch with two positions: a position where it switches the load via the triac , and a position where it disconnects the latter and simultaneously lights a neon indicator 6, with its associated limiting resistor 7, making it easier to find the switch in the dark.

Conventionally, the triac 4 is controlled in phase by a phase shifting circuit comprising, in series, a resistive element 8, 9 composed of a fixed resistance (for example, 2.7 kΩ) and a variable resistance 9 ( for example varying from 0 to 500 kΩ) which can be coupled to switch 5, which is then a limit switch corresponding to the position of maximum resistance. The fixed resis¬ tance 8 is chosen so as to limit the trigger current of the triac at the time of priming of the latter. The phase shifting circuit comprises, associated with the resistive element 8, 9, a capacitive element 10 (for example a capacitor of 0, lμF), so that the midpoint 11 between the resistive element and the capacitive element will have a variable phase shift relative to the power electrodes A1, A2 of the triac, thus making it possible to delay the priming of the latter relative to the start of each half-period and thus to modulate the power delivered to the load 1.

In a manner known per se, the connection between the midpoint 11 of the phase shifting circuit RC and the trigger G of the triac is carried out by means of a diac 12 making it possible to generate current peaks ensuring better priming of the triac.

Preferably, there is also provided on the trigger circuit, in series with the diac, a Zener diode 13 (for example a diode at about 12 V), which ensures complete extinction of the load even if the actual maximum resistance value of variable resistance 9 (for example 470 kΩ) is less than the value nominal maximum (for example, 500 kΩ) for which the circuit has been calculated; this mitigates the consequences of dispersions in the manufacturing characteristics of the variable resistance, dispersions which can be significant. Finally, provision is made for a radio frequency interference suppression circuit comprising an inductor 14 in series with the triac and the load, and a capacitor 15 in parallel on the branch formed by the inductor 14 and the triac 4.

The assembly which we have just described is in itself classic.

As indicated above, the short circuit between the power electrodes A1 and A2 of the triac will be without damaging consequences, as will a short circuit between the trigger G and the power electrode A2; simply, in either of these two cases, it will no longer be possible to vary the power delivered to load 1.

On the other hand, in the event of a short circuit between the trigger and the power electrode Al (short circuit symbolized in 16), the trigger will be brought, via the load 1, to the potential of phase 2 and we will find this potential at midpoint 11, the diac 12 and the Zener diode 13 being both passers-by since they are polarized beyond their limit conduction threshold.

The resistive circuit 8, 9 will therefore receive between its terminals 11, 17 a voltage substantially equal to the mains voltage, therefore considerably greater than that at which it operates usually (of the order of a few volts).

If, at this moment, the variable resistance is in a position corresponding to a high resistance value, this inci¬ dent will not have any harmful consequence, since the effective current passing through the resistive element will remain weak and compatible with the power nominal of the variable resistor 9 (generally 0.5 W). On the other hand, for a range of resistance values between approximately 3 and 10 kΩ (for the values given here by way of example), the intensity passing through the variable resistance 9 and the fixed resistance 8 will become much greater than what can dissipate these two elements, so that they will be destroyed very quickly, heating even often producing the combustion of the printed card with emission of flames or fumes: indeed, for a certain range of resistance values, the current will be sufficient weak not to blow the general protection fuse 3 (which is calibrated to the nominal current passing through the load, which is several amps), but sufficiently strong to cause combustion and destruction of the fixed resistance 8 and / or of the variable resistor 9. To overcome this drawback, the present invention proposes to insert, in series into the trigger circuit (that is to say the branch between point 17 and the triac trigger) a compo¬ sant non Hnaire 18 protecting the circuit against such over-current. In a first embodiment, the component not

Hnea 18 is a resistor with a positive temperature coefficient (PTC), the characteristics of which are chosen so that it can operate as a current monitor: when the overload resulting from short circuit 16 occurs, the PTC heats up due to abrupt increase in intensity and reach its Curie point; its resistance then increases very quickly, becoming much higher than that of the resistive element 8, 9, thus ensuring the protection of the latter.

This mode of réaHsation has the advantage of total reversibility, which is interesting if the cause of the short circuit 16 can be eliminated; However, it has the drawback of requiring an expensive component (CTP) and of having a relatively long response time, of the order of half a second, to heat the CTP sufficiently. In a second embodiment, the non-element

Hnéaire 18 is a fusible element.

This fuse element can be a fast safety fuse of the conventional type (cartridge, tubular, solder fuse, etc.). U has the advantage of simplicity. On the other hand, as the fuse must be chartered at a very low current (typically only 10 mA), this component is relatively expensive and practice shows that the response time is relatively long even for a “fast” type fuse because the values currently commercially available are limited to 10 mA, and such a value is still too high for the protection sought, for which a value of 5 mA or less would be desirable.

Furthermore, very low-current fuses could not be used in dimming circuits for relatively large loads, since they would then be capped at a value too close to the minimum average trigger current required (typically, the current of trigger can reach 50 to 70 mA, depending on the manufacturing).

In a variant - preferential - of implementation, the fusible element is constituted by an incandescent bulb, the characteristics of the filament of which are chosen so that it comes to fly as soon as an overcurrent appears in the trigger circuit.

Thus, in the case of a circuit whose elements have the values indicated above by way of example, it has been observed that a 5 V 15 mA bulb (i.e. a hot resistance of 333 Ω under nominal conditions of operation) met these conditions.

On the other hand, a bulb of the same type of 3 V / 8 mA (i.e. a resistance of 375 Ω under nominal operating conditions) did not fulfill its role as a fuse element: indeed, in the latter case, during from a short circuit in the trigger circuit, the filament was brought to a sharp glow but did not fly, so that the resistive element 8, 9 was destroyed and caused the combustion of the printed circuit board. It is therefore important to make a selection of the am¬ hen used, one of the important parameters being the hot resistance of the filament under the nominal operating conditions: with the examples of numerical value given above, we found that an equivalent resistance greater than approximately 350 Ω prevented the lamp from correctly reheating its role of ele safety, it operates in "incandescent" mode and not in "fuse" mode.

In addition to its very low cost, such an element has the advantage of a very high response speed - greater than that of a PTC or of a conventional “fast” safety fuse.

It thus ensures full compliance with the aforementioned safety standards of the variator according to the invention.

Of course, the values given in the present description are only for information and can vary widely depending on the various values of components used.

It will also be noted that, whatever the mode of realization, the non-linear component 18 can be placed at any location of the trigger control circuit, that is to say at any location. of the branch between common point 17 and the trigger G of the triac. In addition to the position shown in the figure, the element 18 can also, as a variant, be placed for example in series at point 19 (between the midpoint 11 and the diac 12), or even in series at point 20 (just before trigger G).

Claims

1. A power variator circuit, comprising a triac (4) connected in series with a load (1) to be controlled, and a variable series RC phase-shifter circuit (8, 9, 10) whose midpoint (11) between the resistive element (8, 9) and the capacitive element (10) is connected to the trigger (G) of the triac, characterized in that it comprises, in series in the circuit branch comprising the resistive element and the house of said midpoint of triac, a non-Hneal component (18) whose resistance is liable to pass from a minimum value to a maximum value when the current flowing through it increases respectively by a minimum value corresponding to the normal operating conditions of the phase shifting circuit to a maximum value corresponding to a triac short-circuit condition between trigger and power electrode, said minimum resistance value being a value with no significant effect on the operation of the phase-shifting circuit and said maximum value of resistance being a value H miter the current in the resistive element to a value preventing any destruction or deterioration of this one. 2. The dimmer circuit of claim 1, wherein said non-linear component (18) is a PTC resistor. 3. The variator circuit of claim 2, wherein said non-linear component (18) is a fuse element. 4. The variator circuit of claim 3, wherein said fuse element is a fast safety fuse. 5. The dimmer circuit of claim 3, in which said fusible element consists of the filament of an incandescent bulb, a filament whose characteristics, in particular its equivalent resistance under nominal operating conditions, are chosen so that '' it volatilizes in the case of a triac short-circuit condition between trigger and power electrode.